INSR and hepatocellular carcinoma: Of interest, fractionation of the subcellular membrane vesicles of HepG2 hepatoma cells by iodixanol density gradient ultracentrifugation allowed the detection of proIR, IRCTF, and IRNTF in the Golgi apparatus and ER-enriched fractions (Fig. 2G), suggesting that cleavage of endogenous IR occurs in the ESP.